The present invention relates to electrochemical cells, and specifically to a new sealed lead acid cell having improved performance and less susceptibility to shorting due to severe discharge and charge reversal.
A significant improvement in the well-known lead acid cell is the fully sealed lead acid cell which makes use of a cylindrical spiral-wound plate design for high energy density and low internal impedance, and can be used, i.e., charged and discharged, in any position. Such cell typically include spaced-apart positive and negative lead plates having a grid-like construction. The grid structure is filled with the active materials to form either positive (lead dioxide) or negative (sponge lead) electrodes. Sandwiched between the plates is a thin porous separator, the plate-separator assembly being wound into a compact, rugged cylindrical form. The separator electrically isolates the plates, and also functions as an effective wick to retain the cell's electrolyte (an aqueous solution of sulfuric acid) and keep it evenly distributed in the working area. The thin, highly porous separator keeps the ionic path between the plates short and permits rapid diffusion of electrolyte, these factors all contributing to the cell's ability to be discharged at high rates. The typical cell being described also generally includes means such as excess negative plate material, for minimizing the formation of gases in the cell, and a resealable vent for releasing internal pressure in the cell should unwanted gases be generated.
While the above-described cell represents a significant improvement over prior lead acid cells, it is desirable to further improve its performance and lengthen its life cycle, especially when the cell is subjected to extreme conditions. For example, when a lead acid cell is allowed to stand on open circuit, a slow electrochemical discharge occurs, the rate of self-discharge depending on the cell temperature and its state of charge. If a cell is allowed to self-discharge completely, i.e., until substantially all of the sulfate ion in the electrolyte has reacted with the plate materials, the lead sulfate becomes slightly soluble in the very dilute electrolyte and is free to diffuse into the separator between the plates. Attempting to recharge the cell in this condition may result in the formation of lead dendrites in the separator between the plates, eventually shorting the cell and ending its useful life. Similarly, where a discharged lead-acid cell is for one reason or another connected to a charge in reverse, the cell will accept a charge (the positive plate becomes a negative plate and vice versa), but is vulnerable to being shorted out by deposits of metallic lead and lead sulfate in the separator.
In addition, it has been found that there are certain problems associated with winding the plate-separator assembly of known cells. More particularly, in the manufacture of electrolytic cells or batteries, the positive and the negative plate materials are formed as strips which are then cut into cell strip lengths and wound, with separator material therebetween, into a coil. The coil is then inserted into a preformed cylindrical container. Electrolyte is added to the container and the container is closed and sealed. The sealed cell is then charged. The coiled positive plate is connected to the positive terminal and the coiled negative plate is connected to the negative terminal. Typically, the plate strips and separators are wound into the composite coiled assembly about a mandrel with a kiss roller or the inner surface of a cylindrical winding nest engaging one side of the coil as it is being wound. One of the difficulties with this arrangement, however, is that the forces required for winding are applied through the mandrel directly to the strips being wound. Thus, tensile forces are applied to the separators as the separators and plates are pulled by the arbor into the composite, coiled assembly. Because the separators have a low tensile strength, these winding forces can cause damage to and/or breakage of the separators, and can result in a shorted coil, which, of course, adversely affects overall cell quality.
Accordingly, it is an object of the present invention to provide a new and improved structure and method of manufacturing a sealed lead-acid cell which has an extended useful life, and which is less vulnerable to electrical shorts as a result of the cell being subjected to extreme conditions such as overdischarge or charge reversal.
It is another object of the present invention to provide a sealed lead-acid cell having an electrode assembly which may be wound automatically into a cylindrical coil without damage to the separators.